1. Field of the Invention
This invention relates to meltblown nonwoven webs, and, in particular, to meltblown nonwoven webs made from a blend of an epoxy resin and a polycaprolactone ("PCL") polymer.
2. Description of the Prior Art
In the 1950's, the United States Naval Research Laboratory developed the meltblowing process for producing nonwoven webs from thermoplastic resins. See Wente et al., "Manufacture of Superfine Organic Fibers," Naval Research Laboratory Report No. 111437, Naval Research Laboratory, Washington, D.C., May 25, 1954; and Wente, Van. A., "Superfine Thermoplastic Fibers," Industrial and Engineering Chemistry, Vol. 48, No. 8, pages 1342-1346 (1956).
In overview, the process involves forming relatively small diameter fibers from the thermoplastic resin and then randomly depositing those fibers on, for example, a moving screen to form the nonwoven web. More particularly, the process comprises heating the resin to a molten state and then extruding the molten resin as threads or filaments from a die having a plurality of linearly arranged small diameter capillaries or orifices. The molten filaments exit the die into a high velocity stream of a heated gas which usually is air. The heated gas serves to attenuate, or draw, the filaments to form fibers having diameters which are less than the diameters of the capillaries of the die. The fibers thus obtained are usually deposited in a random fashion on a moving porous collecting device, such as a screen or wire, thereby resulting in the formation of the desired nonwoven web.
General discussions of the meltblowing process can be found in the Wente references referred to above, as well as in Buntin et al., "Melt Blowing--A One-Step Web Process for New Nonwoven Products," Journal of the Technical Association of the Pulp and Paper Industry, Vol. 56, No. 4, pages 74-77 (1973), the relevant portions of which are incorporated herein by reference. Specific examples of the technique can be found in U.S. Pat. No. 3,016,599 to Perry, Jr., U.S. Pat. No. 3,704,198 to Prentice, U.S. Pat. No. 3,755,527 to Keller et al., U.S. Pat. No. 3,849,241 to Butin et al., U.S. Pat. No. 3,978,185 to Buntin et al., U.S. Pat. No. 4,100,324 to Anderson et al., U.S. Pat. No. 4,118,531 to Hauser, U.S. Pat. No. 4,663,220 to Wisneski et al., and U.S. Pat. No. 4,820,577 to Morman et al., the relevant portions of which are also incorporated herein by reference.
Although meltblowing has been performed with a variety of thermoplastic resins, to date, the technique has not been successfully applied to epoxy resins. Indeed, in the course of developing the present invention, attempts were made to meltblow lower molecular weight epoxy resins. In each case, the result was a glassy and very brittle non-bonded web, totally unsuitable for commercial use, e.g., as a nonwoven fabric.
This inability to produce useful products through the meltblowing of epoxy resins is a significant deficit in the art both because meltblowing is a highly effective and economical method for producing nonwoven products and because epoxy-based materials have excellent physical and chemical properties, including toughness, good dielectric properties, and good corrosion and chemical resistance. As discussed in detail below, the present invention addresses this problem in the art by providing blends of epoxy resins, specifically, blends with PCL polymers, which can be meltblown to form nonwoven webs. Surprisingly, the webs have enhanced physical and chemical properties characteristic of an epoxy resin, and yet are neither glassy nor brittle.
Some combinations of polycaprolactones with epoxy resins have been reported in the literature. For example, Union Carbide Corporation's 1988 product brochure entitled "TONE.RTM. Polymers P-300 and P-700 High Molecular Weight Caprolactone Polymers" (Brochure No. F-60456 at page 9), lists epoxies as one type of a number of polymers which are mechanically compatible, but not miscible, with polycaprolactones.
Similarly, U.S. Pat. No. 4,567,216 to Qureshi et al., describes an epoxy resin system which comprises an epoxy resin, specifically bis(2,3-epoxycylopentyl) ether, a hardener, and a thermoplastic polymer which can be a polycaprolactone. The composition is used to prepare fiber-reinforced composites for making aircraft parts and the like. Along these same lines, U.S. Pat. No. 4,540,729 to Williams discloses a molding composition which comprises a polyethylene terephthalate polyester, a nucleant for crystallizing the polyester, an epoxidised unsaturated triglyceride, and a polycaprolactone. See also Japanese Patent Publication No. 59/030,817 [Chem. Abstr., 101:92135c (1984)] which describes reacting polycaprolactones with dicarboxylic anhydrides and mixing the resulting adducts with an alicyclic epoxy resin to produce a molding composition. Cured resin moldings made from the composition are said to have excellent flexibility, toughness, and electrical properties, making them suitable for use as electrical insulating material.
Lactones have also been used to modify polymers, including polymers containing epoxy groups. Thus, U.S. Pat. No. 4,475,998 to Okitsu et al. describes the preparation of a lactone-modified epoxy (meth)acrylate resin which is combined with a vinyl compound having an ethylenically unsaturated bond and a photosensitizer to produce a hardenable resin composition. The addition of the lactone is said to improve the flexibility of the epoxy (meth)acrylate resin. Similarly, Japanese Patent Publication No. 61/004,773 [Chem. Abstr., 105:61500w (1986)] discloses a prepregnated cloth for use as an insulating material for electrical machines wherein the resin used to impregnate the cloth includes, among other ingredients, an epoxy resin and a caprolactone-modified epoxy resin. See also Japanese Patent Publication No. 61/241,321 [Chem. Abstr., 106:196976x (1987)] which discloses the preparation of spiro ortho esters by reacting an epoxy compound with a lactone and the use of such compounds to surface treat carbon substances, such as graphites; U.K. Patent Application Serial No. 2,158,081 which describes a graft polymer of cellulose and caprolactone which can be used as a coating resin or a molding material; and U.K. Patent No. 1,153,364 which discloses a process for the production of .epsilon.-caprolactone and states that the resulting product can be polymerized or copolymerized with epoxides to form synthetic resins and fibers.
In addition to the above references, a variety of theoretical studies have been performed on blends and mixtures of polycaprolactone polymers with other polymers, including poly(vinyl chloride), bisphenol epoxy resins prepared from bisphenol A and epichlorohydrin, cellulosic polymers, polyepichlorohydrin, a chlorinated polyether, poly(vinyl acetate), polystyrene, poly(methyl methacrylate), poly(vinyl butyral), poly(vinyl alkyl ethers), polysulfone, polycarbonates, natural and synthetic rubbers, polyethylene, chlorinated polyethylene, polypropylene, and polyurethanes. See Koleske, J. V., "Blends Containing Poly(.epsilon.-Caprolactone) and Related Polymers" in "Polymer Blends", Vol. 2, pages 369-389 (1978); Kalfoglou, N. K., "Mechanical and Thermal Characterization of Poly-(.epsilon.-Caprolactone)-Chlorinated Polyethylene Blends" in Seferis, J. C. and Theocans, P. S., Editors, "Interrelations between Processing Structure and Properties of Polymeric Materials", pages 481-494 (1984); Prud'homme, R. E., "Miscibility Phenomena in Polyester/Chlorinated Polymer Blends," Polymer Engineering and Science, 22, No. 2, pages 90-95 (1982); Coleman, M. M. and Zarian, J., "Fourier-Transform Infrared Studies of Polymer Blends. II. Poly(.epsilon.-Caprolactone)-Poly(Vinyl Chloride) System," Journal of Polymer Science: Polymer Physics Edition, 17, pages 837-850 (1979); Robeson, L. M., "Crystallization Kinetics of Poly-.epsilon.-Caprolactone from Poly-.epsilon.-Caprolactone/Poly(vinyl Chloride) Solutions," Journal of Applied Polymer Science, 17, pages 3607-3617 (1973); Hubbell, D. S. and Cooper, S. L., "The Physical Properties and Morphology of Poly-.epsilon.-Caprolactone Polymer Blends," Journal of Applied Polymer Science, 21, pages 3035-3061 (1977); Koleske, J. V. and Lundberg, R. D., "Lactone Polymers. I. Glass Transition Temperature of Poly-.epsilon.-Caprolactone by Means of Compatible Polymer Mixtures," Journal of Polymer Science, Part A-2, 7, pages 795-807 (1969); Khambatta, F. B., Warner, F., Russell, T., and Stein, R. S., "Small-Angle X-Ray and Light Scattering Studies of the Morphology of Blends of Poly(.epsilon.-Caprolactone) with Poly(vinyl Chloride)," Journal of Polymer Science: Polymer Physics Edition, 14, pages 1391-1424 (1976); and Hubbell, D. S. and Cooper, S. L., "Segmental Orientation, Physical Properties, and Morphology of Poly-.epsilon.-Caprolactone Blends," in S. L. Cooper and G. M. Estes, Editors, "Multiphase Polymers," Advances in Chemistry Series 176, American Chemical Society, Washington, D.C., 1979, pp. 517-528. See also U.S. Pat. No. 3,901,838 to Clendinning et al.; U.S. Pat. No. 4,064,195 to Baron et al.; U.S. Pat. No. 3,925,504 to Koleske et al.; U.S. Pat. No. 3,632,687 to Walter et al.; U.S. Pat. No. 3,734,979 to Koleske et al.; and U.S. Pat. No. 3,781,381 to Koleske et al.
Notwithstanding the extensive efforts that have gone into the study of blends of polycaprolactone polymers, none of the foregoing references discloses or in any way suggests the use of these polymers in the preparation of meltblown webs from epoxy resins.